Phosphatidylinositol 3-kinase activation is mediated by high-affinity interactions between distinct domains within the p110 and p85 subunits

SUMOylation modulates the stability and function of PI3K-p110β

Class I PI3K are heterodimers composed of a p85 regulatory subunit and a p110 catalytic subunit involved in multiple cellular functions. Recently, the catalytic subunit p110β has emerged as a class I PI3K isoform playing a major role in tumorigenesis. Understanding its regulation is crucial for the control of the PI3K pathway in p110β-driven cancers. Here we sought to evaluate the putative regulation of p110β by SUMO. Our data show that p110β can be modified by SUMO1 and SUMO2 in vitro, in transfected cells and under completely endogenous conditions, supporting the physiological relevance of p110β SUMOylation. We identify lysine residue 952, located at the activation loop of p110β, as essential for SUMOylation. SUMOylation of p110β stabilizes the protein increasing its activation of AKT which promotes cell growth and oncogenic transformation. Finally, we show that the regulatory subunit p85β counteracts the conjugation of SUMO to p110β. In summary, our data reveal that SUMO is a novel p110β interacting partner with a positive effect on the activation of the PI3K pathway.

Interaction of Calmodulin with the cSH2 Domain of the p85 Regulatory Subunit

Calmodulin (CaM) is a calcium sensor protein that directly interacts with the dual-specificity (lipid and protein) kinase PI3Kα through the SH2 domains of the p85 regulatory subunit. In adenocarcinomas, the CaM interaction removes the autoinhibition of the p110 catalytic subunit of PI3Kα, leading to activation of PI3Kα and promoting cell proliferation, survival, and migration. Here we demonstrate that the cSH2 domain of p85α engages its two CaM-binding motifs in the interaction with the N- and C-lobes of CaM as well as the flexible central linker, and our nuclear magnetic resonance experiments provide structural details. We show that in response to binding CaM, cSH2 exposes its tryptophan residue at the N-terminal region to the solvent. Because of the flexible nature of both CaM and cSH2, multiple binding modes of the interactions are possible. Binding of CaM to the cSH2 domain can help release the inhibition imposed on the p110 subunit, similar to the binding of the phosphorylated motif of RTK, or phosphorylated CaM (pCaM), to the SH2 domains. Amino acid sequence analysis shows that CaM-binding motifs are common in SH2 domains of non-RTKs. We speculate that CaM can also activate these kinases through similar mechanisms.

Regulation of therapeutic resistance in cancers by receptor tyrosine kinases

In response to DNA damage lesions due to cellular stress, DNA damage response (DDR) pathways are activated to promote cell survival and genetic stability or unrepaired lesion-induced cell death. Current cancer treatments predominantly utilize DNA damaging agents, such as irradiation and chemotherapy drugs, to inhibit cancer cell proliferation and induce cell death through the activation of DDR. However, a portion of cancer patients is reported to develop therapeutic resistance to these DDR-inducing agents. One significant resistance mechanism in cancer cells is oncogenic kinase overexpression, which promotes cell survival by enhancing DNA damage repair pathways and evading cell cycle arrest. Among the oncogenic kinases, overexpression of receptor tyrosine kinases (RTKs) is reported in many of solid tumors, and numerous clinical trials targeting RTKs are currently in progress. As the emerging trend in cancer treatment combines DNA damaging agents and RTK inhibitors, it is important to understand the substrates of RTKs relative to the DDR pathways. In addition, alteration of RTK expression and their phosphorylated substrates can serve as biomarkers to stratify patients for combination therapies. In this review, we summarize the deleterious effects of RTKs on the DDR pathways and the emerging biomarkers for personalized therapy.

Evolutionary history of phosphatidylinositol- 3-kinases: ancestral origin in eukaryotes and complex duplication patterns

BACKGROUND

Phosphatidylinositol-3-kinases (PI3Ks) are a family of eukaryotic enzymes modifying phosphoinositides in phosphatidylinositols-3-phosphate. Located upstream of the AKT/mTOR signalling pathway, PI3Ks activate secondary messengers of extracellular signals. They are involved in many critical cellular processes such as cell survival, angiogenesis and autophagy. PI3K family is divided into three classes, including 14 human homologs. While class II enzymes are composed of a single catalytic subunit, class I and III also contain regulatory subunits. Here we present an in-depth phylogenetic analysis of all PI3K proteins.

RESULTS

We confirmed that PI3K catalytic subunits form a monophyletic group, whereas regulatory subunits form three distinct groups. The phylogeny of the catalytic subunits indicates that they underwent two major duplications during their evolutionary history: the most ancient arose in the Last Eukaryotic Common Ancestor (LECA) and led to the emergence of class III and class I/II, while the second - that led to the separation between class I and II - occurred later, in the ancestor of Unikonta (i.e., the clade grouping Amoebozoa, Fungi, and Metazoa). These two major events were followed by many lineage specific duplications in particular in vertebrates, but also in various protist lineages. Major loss events were also detected in Vidiriplantae and Fungi. For the regulatory subunits, we identified homologs of class III in all eukaryotic groups indicating that, for this class, both the catalytic and the regulatory subunits were presents in LECA. In contrast, homologs of the regulatory class I have a more recent origin.

CONCLUSIONS

The phylogenetic analysis of the PI3K shed a new light on the evolutionary history of these enzymes. We found that LECA already contained a PI3K class III composed of a catalytic and a regulatory subunit. Absence of class II regulatory subunits and the recent origin of class I regulatory subunits is puzzling given that the class I/II catalytic subunit was present in LECA and has been conserved in most present-day eukaryotic lineages. We also found surprising major loss and duplication events in various eukaryotic lineages. Given the functional specificity of PI3K proteins, this suggests dynamic adaptation during the diversification of eukaryotes.

Exploring the role of two interacting phosphoinositide 3-kinases of Haemonchus contortus

Background

Phosphoinositide 3-kinases (PI3Ks) are relatively conserved and important intracellular lipid kinases involved in signalling and other biological pathways. In the free-living nematode Caenorhabditis elegans, the heterodimeric form of PI3K consists of catalytic (AGE-1) and regulatory (AAP-1) subunits. These subunits are key components of the insulin-like signalling pathway and play roles in the regulation of the entry into and exit from dauer. Although, in parasitic nematodes, similar components are proposed to regulate the transition from free-living or arrested stages to parasitic larvae, nothing is known about PI3Ks in relation to the transition of third-stage larvae (L3s) to parasitism in Haemonchus contortus.

Methods

An integrated molecular approach was used to investigate age-1 and aap-1 of H. contortus (Hc-age-1 and Hc-aap-1) in C. elegans.

Results

The two genes Hc-age-1 and Hc-aap-1 were transcribed in all life stages, with the highest levels in the egg, infective L3 and adult female of H. contortus. The expression of these genes was localized to the intestine, contrasting the pattern of their orthologues in C. elegans (where they are expressed in both head neurons and the intestine). The yeast two-hybrid analysis demonstrated that the adaptor-binding domain of Hc-AGE-1 interacted strongly with the Hc-AAP-1; however, this complex did not rescue the function of its orthologue in age-1-deficient C. elegans.

Conclusions

This is the first time that the PI3K-encoding genes have been characterized from a strongylid parasitic nematode. The findings provide insights into the role of the PI3K heterodimer represented by Hc-age-1 and Hc-aap-1 in the developmental biology of H. contortus.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-014-0498-2) contains supplementary material, which is available to authorized users.

Molecular mechanisms and functional implications of polarized actin remodeling at the T cell immunological synapse

Transient,specialized cell-cell interactions play a central role in leukocyte function by enabling specific intercellular communication in the context of a highly dynamic systems level response. The dramatic structural changes required for the formation of these contacts are driven by rapid and precise cytoskeletal remodeling events. In recent years, the immunological synapse that forms between a T lymphocyte and its antigen-presenting target cell has emerged as an important model system for understanding immune cell interactions. In this review, we discuss how regulators of the cortical actin cytoskeleton control synaptic architecture and in this way specify T cell function.

P42 Ebp1 regulates the proteasomal degradation of the p85 regulatory subunit of PI3K by recruiting a chaperone-E3 ligase complex HSP70/CHIP

The short isoform of ErbB3-binding protein 1 (Ebp1), p42, is considered to be a potent tumor suppressor in a number of human cancers, although the mechanism by which it exerts this tumor-suppressive activity is unclear. Here, we report that p42 interacts with the cSH2 domain of the p85 subunit of phosphathidyl inositol 3-kinase (PI3K), leading to inhibition of its lipid kinase activity. Importantly, we found that p42 induces protein degradation of the p85 subunit and further identified HSP70/CHIP complex as a novel E3 ligase for p85 that is responsible for p85 ubiquitination and degradation. In this process, p42 couples p85 to the HSP70/CHIP-mediated ubiquitin–proteasomal system (UPS), thereby promoting a reduction of p85 levels both in vitro and in vivo. Thus, the tumor-suppressing effects of p42 in cancer cells are driven by negative regulation of the p85 subunit of PI3K.

Activation of PI3Kα by physiological effectors and by oncogenic mutations: structural and dynamic effects

PI3Kα, a heterodimeric lipid kinase, catalyzes the conversion of phosphoinositide-4,5-bisphosphate (PIP2) to phosphoinositide-3,4,5-trisphosphate (PIP3), a lipid that recruits to the plasma membrane proteins that regulate signaling cascades that control key cellular processes such as cell proliferation, carbohydrate metabolism, cell motility, and apoptosis. PI3Kα is composed of two subunits, p110α and p85, that are activated by binding to phosphorylated receptor tyrosine kinases (RTKs) or their substrates. The gene coding for p110α, PIK3CA, has been found to be mutated in a large number of tumors; these mutations result in increased PI3Kα kinase activity. The structure of the complex of p110α with a fragment of p85 containing the nSH2 and the iSH2 domains has provided valuable information about the mechanisms underlying the physiological activation of PI3Kα and its pathological activation by oncogenic mutations. This review discusses information derived from x-ray diffraction and theoretical calculations regarding the structural and dynamic effects of mutations in four highly mutated regions of PI3K p110α, as well as the proposed mechanisms by which these mutations increase kinase activity. During the physiological activation of PI3Kα, the phosphorylated tyrosine of RTKs binds to the nSH2 domain of p85, dislodging an inhibitory interaction between the p85 nSH2 and a loop of the helical domain of p110α. Several of the oncogenic mutations in p110α activate the enzyme by weakening this autoinhibitory interaction. These effects involve structural changes as well as changes in the dynamics of the enzyme. One of the most common p110α mutations, H1047R, activates PI3Kα by a different mechanism: it increases the interaction of the enzyme with the membrane, maximizing the access of the PI3Kα to its substrate PIP2, a membrane lipid.

Annular PIP3 accumulation controls actin architecture and modulates cytotoxicity at the immunological synapse

In T cells, PI3K activation in the periphery of the immune synapse leads to PIP₃ accumulation that promotes actin polymerization in a pathway important for cytotoxic function.

The immunological synapse formed by a T lymphocyte on the surface of a target cell contains a peripheral ring of filamentous actin (F-actin) that promotes adhesion and facilitates the directional secretion of cytokines and cytolytic factors. We show that growth and maintenance of this F-actin ring is dictated by the annular accumulation of phosphatidylinositol trisphosphate (PIP₃) in the synaptic membrane. PIP₃ functions in this context by recruiting the exchange factor Dock2 to the periphery of the synapse, where it drives actin polymerization through the Rho-family GTPase Rac. We also show that synaptic PIP₃ is generated by class IA phosphoinositide 3-kinases that associate with T cell receptor microclusters and are activated by the GTPase Ras. Perturbations that inhibit or promote PIP₃-dependent F-actin remodeling dramatically affect T cell cytotoxicity, demonstrating the functional importance of this pathway. These results reveal how T cells use lipid-based signaling to control synaptic architecture and modulate effector responses.

Activation of PI3K/Akt signaling by n-terminal SH2 domain mutants of the p85α regulatory subunit of PI3K is enhanced by deletion of its c-terminal SH2 domain

The phosphoinositide 3-kinase (PI3K) is frequently activated in human cancer cells due to gain of function mutations in the catalytic (p110) and the regulatory (p85) subunits. The regulatory subunit consists of an SH3 domain and two SH2 domains. An oncogenic form of p85α named p65 lacking the c-terminal SH2 domain (cSH2) has been cloned from an irradiation-induced murine thymic lymphoma and transgenic mice expressing p65 in T lymphocytes develop a lymphoproliferative disorder. We have recently detected a c-terminal truncated form of p85α named p76α in a human lymphoma cell line lacking most of the cSH2 domain due to a frame shift mutation. Here, we report that the deletion of the cSH2 domain enhances the activating effects of the n-terminal SH2 domain (nSH2) mutants K379E and R340E on the PI3K/Akt pathway and micro tumor formation in a focus assay. Further analysis revealed that this transforming effect is mediated by activation of the catalytic PI3K isoform p110α and downstream signaling through mTOR. Our data further support a mechanistic model in which mutations of the cSH2 domain of p85α can abrogate its negative regulatory function on PI3K activity via the nSH2 domain of p85α.

The regulation of class IA PI 3-kinases by inter-subunit interactions

Phosphoinositide 3-kinases (PI 3-kinases) are activated by growth factor and hormone receptors, and regulate cell growth, survival, motility, and responses to changes in nutritional conditions (Engelman et al. 2006). PI 3-kinases have been classified according to their subunit composition and their substrate specificity for phosphoinositides (Vanhaesebroeck et al. 2001). The class IA PI 3-kinase is a heterodimer consisting of one regulatory subunit (p85α, p85β, p55α, p50α, or p55γ) and one 110-kDa catalytic subunit (p110α, β or δ). The Class IB PI 3-kinase is also a dimer, composed of one regulatory subunit (p101 or p87) and one catalytic subunit (p110γ) (Wymann et al. 2003). Class I enzymes will utilize PI, PI[4]P, or PI[4,5]P2 as substrates in vitro, but are thought to primarily produce PI[3,4,5]P3 in cells.The crystal structure of the Class IB PI 3-kinase catalytic subunit p110γ was solved in 1999 (Walker et al. 1999), and crystal or NMR structures of the Class IA p110α catalytic subunit and all of the individual domains of the Class IA p85α regulatory subunit have been solved (Booker et al. 1992; Günther et al. 1996; Hoedemaeker et al. 1999; Huang et al. 2007; Koyama et al. 1993; Miled et al. 2007; Musacchio et al. 1996; Nolte et al. 1996; Siegal et al. 1998). However, a structure of an intact PI 3-kinase enzyme has remained elusive. In spite of this, studies over the past 10 years have lead to important insights into how the enzyme is regulated under physiological conditions. This chapter will specifically discuss the regulation of Class IA PI 3-kinase enzymatic activity, focusing on regulatory interactions between the p85 and p110 subunits and the modulation of these interactions by physiological activators and oncogenic mutations. The complex web of signaling downstream from Class IA PI 3-kinases will be discussed in other chapters in this volume.

PI 3-kinase regulatory subunits as regulators of the unfolded protein response

The endoplasmic reticulum (ER) consists of an interconnected, membranous network that is the major site for the synthesis and folding of integral membrane and secretory proteins. Within the ER lumen, protein folding is facilitated by molecular chaperones and a variety of enzymes that ensure that polypeptides obtain their appropriate, tertiary conformation (Dobson, C. M. (2004). Principles of protein folding, misfolding and aggregation. Semin. Cell Dev. Biol. 15, 3-16; Ni, M., and Lee, A. S. (2007). ER chaperones in mammalian development and human diseases. FEBS Lett. 581, 3641-3651.). Physiological conditions that increase protein synthesis or stimuli that disturb the processes by which proteins obtain their native conformation, create an imbalance between the protein-folding demand and capacity of the ER. This results in the accumulation of unfolded or improperly folded proteins in the ER lumen and a state of ER stress. The cellular response, referred to as the unfolded protein response (UPR), results in activation of three linked signal transduction pathways: PKR-like kinase (PERK), inositol requiring 1 α (IRE1α), and activating transcription factor 6α (ATF6α) (Ron, D., and Walter, P. (2007). Signal integration in the endoplasmic reticulum unfolded protein response. Nat. Rev. Mol. Cell. Biol. 8, 519-529; Schroder, M., and Kaufman, R. (2005). ER stress and the unfolded protein response. Mutat. Res./Fundam. Mol. Mech. Mutagen. 569, 29-63.). Collectively, the combined actions of these signaling cascades serve to reduce ER stress through attenuation of translation to reduce protein synthesis and through activation of transcriptional programs that ultimately serve to increase ER protein-folding capacity. Recently, we and Park et al. have characterized a novel function for the p85α and p85β subunits as modulators of the UPR by virtue of their ability to facilitate the nuclear entry of XBP-1s following induction of ER stress (Park, S. W., Zhou, Y., Lee, J., Lu, A., Sun, C., Chung, J., Ueki, K., and Ozcan, U. (2010). Regulatory subunits of PI3K, p85alpha and p85 beta, interact with XBP1 and increase its nuclear translocation. Nat. Med. 16, 429-437; Winnay, J. N., Boucher, J., Mori, M. A., Ueki, K., and Kahn, C. R. (2010). A regulatory subunit of phosphoinositide 3-kinase increases the nuclear accumulation of X-box-binding protein-1 to modulate the unfolded protein response. Nat. Med. 16, 438-445.). This chapter describes the recently elucidated role for the regulatory subunits of PI 3-kinase as modulators of the UPR and provides methods to measure UPR pathway activation.

Somatic mutations in p85alpha promote tumorigenesis through class IA PI3K activation

Members of the mammalian phosphoinositide-3-OH kinase (PI3K) family of proteins are critical regulators of various cellular process including cell survival, growth, proliferation, and motility. Oncogenic activating mutations in the p110alpha catalytic subunit of the heterodimeric p110/p85 PI3K enzyme are frequent in human cancers. Here we show the presence of frequent mutations in p85alpha in colon cancer, a majority of which occurs in the inter-Src homology-2 (iSH2) domain. These mutations uncouple and retain p85alpha's p110-stabilizing activity, while abrogating its p110-inhibitory activity. The p85alpha mutants promote cell survival, AKT activation, anchorage-independent cell growth, and oncogenesis in a p110-dependent manner.

Ligand-induced EpoR internalization is mediated by JAK2 and p85 and is impaired by mutations responsible for primary familial and congenital polycythemia

Epo-induced endocytosis of EpoR plays important roles in the down-regulation of EpoR signaling and is the primary means that regulates circulating Epo concentrations. Here we show that cell-surface EpoR is internalized via clathrin-mediated endocytosis. Both JAK2 kinase activity and EpoR cytoplasmic tyrosines are important for ligand-dependent EpoR internalization. Phosphorylated Y429, Y431, and Y479 in the EpoR cytoplasmic domain bind p85 subunit of PI3 kinase on Epo stimulation and individually are sufficient to mediate Epo-dependent EpoR internalization. Knockdown of p85alpha and p85beta or expression of their dominant-negative forms, but not inhibition of PI3 kinase activity, dramatically impaired EpoR internalization, indicating that p85alpha and p85beta may recruit proteins in the endocytic machinery on Epo stimulation. Furthermore, mutated EpoRs from primary familial and congenital polycythemia (PFCP) patients lacking the 3 important tyrosines do not bind p85 or internalize on stimulation. Addition of residues encompassing Y429 and Y431 to these truncated receptors restored p85beta binding and Epo sensitivity. Our results identify a novel PI3 kinase activity-independent function of p85 in EpoR internalization and support a model that defects of internalization in truncated EpoRs from PFCP patients contribute to Epo hypersensitivity and prolonged signaling.

Regulation of class IA PI3Ks

Class IA PI3Ks (phosphoinositide 3-kinases) regulate a wide range of cellular responses through the production of PI(3,4,5)P(3) (phosphatidylinositol 3,4,5-trisphosphate) in cellular membranes. They are activated by receptor tyrosine kinases, by Ras and Rho family GTPases, and in some cases by G(betagamma) subunits from trimeric G-proteins. Crystallographic studies on the related class IB PI3Kgamma, and biochemical and structural studies on the class IA PI3Ks, have led to new insights into how these critical enzymes are regulated in normal cells and how mutations can lead to their constitutive activation in transformed cells. The present paper will discuss recent studies on the regulation of class I (p85/p110) PI3Ks, with a focus on the role of SH2 domains (Src homology 2 domains) in the p85 regulatory subunit in modulating PI3K activity.

Mutations in the inter-SH2 domain of the regulatory subunit of phosphoinositide 3-kinase: effects on catalytic subunit binding and holoenzyme function

Class IA phosphoinositide 3-kinases (PI3Ks) represent a group of heterodimeric lipid kinases with important functions in cellular signal transduction. The regulatory p85 subunit constitutively binds to the catalytic p110 subunit and mediates the recruitment of the heterodimer to various membrane-localized proteins upon activation by a vast array of stimuli. The functional characterization of protein domains that mediate p85 function has been hampered by a lack of structural data. Therefore, we investigated a 35-aa region in the inter-SH2 domain of p85, reported to be necessary for binding of p110, by site-directed mutagenesis and evaluated the importance of individual amino acids for PI3K heterodimer formation. This approach led to the identification of an 11-aa region required for p110 binding in vitro and mesoderm induction during early Xenopus development in vivo. Further analyses revealed two pairs of hydrophobic amino acids within this region, which are particularly important for high-affinity intersubunit interaction. Thus, our data provide further insight into the molecular mechanisms of PI3K intersubunit interaction and led to the identification of new p85 mutant proteins with varying degrees of dominant-negative effects that will be helpful for future PI3K-related research.

The iSH2 domain of PI 3-kinase is a rigid tether for p110 and not a conformational switch

Class IA PI 3-kinases are heterodimeric proteins with distinct catalytic (p110) and regulatory (p85) subunits. The minimal fragment of p85 capable of regulating p110 activity (p85ni) is the N-terminal SH2 domain linked to the iSH2 coiled-coil domain. We used cysteine mutagenesis and (14)C-NEM-labeling to show that the p110-binding site in the iSH2 domain includes two regions: residues 482-484 and 532-541. These regions are adjacent to each other in the three-dimensional structural model of the iSH2 domain, and define a coherent binding site. We then used spin labeling and EPR spectroscopy to demonstrate that the conformation of the iSH2 domain is unaffected by binding to the N-terminal fragment of p110 (residues 1-108), and/or by phosphopeptide binding to p85ni/p110(1-108) heterodimers. Finally, we show that the cSH2 domain cannot substitute for the nSH2 domain with regard to inhibition of p110. These data support a model in which the iSH2 domain is a rigid tether for p110, and regulation of p85/p110 is mediated by nSH2-p110 contacts.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

Insulin receptor substrate and p55 orthologous adaptor proteins function in the Caenorhabditis elegans daf-2/insulin-like signaling pathway

An insulin-like signaling pathway regulates development and lifespan in Caenorhabditis elegans. Genetic screens that identified many components of the C. elegans insulin pathway did not identify homologs of insulin receptor substrates or the phosphoinositide 3-kinase (PI3K) adaptor/regulatory subunit, which are both required for signaling by mammalian insulin/insulin-like growth factor I pathways. The C. elegans genome contains one homolog of each protein. The C. elegans versions of insulin receptor substrate (IST-1) and PI3K p50/p55 (AAP-1) share moderate sequence similarity with their vertebrate and Drosophila counterparts. Genetic experiments show that ist-1 and aap-1 potentiate C. elegans insulin-like signaling, although they are not required for signaling in the pathway under most conditions. Worms lacking AAP-1 activity because of the mutation aap-1(m889) constitutively arrest development at the dauer larval stage when raised at high temperatures. aap-1 mutants also live longer than wild-type animals, a phenotype observed in other C. elegans mutants with defects in DAF-2 signaling. Interestingly, IST-1 appears to be required for signaling through a pathway that may act in parallel to AGE-1/PI3K.

How do inhibitory phosphatases work?

We present a hypothesis regarding the mode of induction of the inhibitory phosphatases SHP-1 and SHIP in hematopoietic cells. One mode is a general one in which the phosphatase regulates but does not abort signal transduction and biology. Regulator phosphatases are induced by directly or indirectly engaging the amino acid motifs present in the activating receptor, and act to control the biochemical and biological output. The other mode of induction is a specific one, which critically involves paired co-clustering of activating and inhibitory receptors. Phosphatases working in this way act only under conditions of paired co-clustering of activating and inhibitory receptors, and directly bind amino acid motifs present in the inhibitory receptor. However, this mode of induction is apparently more efficient, as cellular activation is completely aborted. This review presents several examples of each mode of inhibition and speculates on their mechanisms.

The p85 regulatory subunit controls sequential activation of phosphoinositide 3-kinase by Tyr kinases and Ras

Class IA phosphoinositide 3-kinase (PI3K) is a heterodimer composed of a p85 regulatory and a p110 catalytic subunit that regulates a variety of cell responses, including cell division and survival. PI3K is activated following Tyr kinase stimulation and by Ras. We found that the C-terminal region of p85, including the C-Src homology 2 (C-SH2) domain and part of the inter-SH2 region, protects the p110 catalytic subunit from Ras-induced activation. Although the p110 activity associated with a C-terminal p85 deletion mutant increased significantly in the presence of an active form of Ras, purified wild type p85-p110 was only slightly stimulated by active Ras. Nonetheless, incubation of purified p85-p110 with Tyr-phosphorylated peptides, which mimic the activated platelet-derived growth factor receptor, restored Ras-induced p85-p110 activation. In conclusion, p85 inhibits p110 activation by Ras; this blockage is released by Tyr kinase stimulation, showing that the classical mechanism of class IA PI3K stimulation mediated by Tyr kinases also regulates Ras-induced PI3K activation.

Negative regulation of urokinase-type plasminogen activator production through FGF-2-mediated activation of phosphoinositide 3-kinase

Activation of phosphoinositide 3-kinase (PI3-kinase) is involved in many cellular responses. FGF-2 is one of the potent inducers of urokinase-type plasminogen activator (uPA) production in endothelial cells. However, little is known about the molecular mechanisms underlying FGF-2-mediated uPA production. Here we examined the signal transduction pathways involved in the regulation of uPA production by FGF-2-treatment. FGF-2 potently upregulated uPA production in murine brain capillary endothelial cells (IBE cells), as well as porcine aortic endothelial (PAE) cells and L6 myoblasts ectopically expressing FGFR1. PI3-kinase inhibitors, wortmannin and LY294002, both enhanced FGF-2-dependent uPA production by these cells. Stable expression of activated mutant p110alpha catalytic subunit of PI3-kinase into IBE cells decreased FGF-2-mediated uPA production, suggesting that PI3-kinase exhibited the negative regulatory effect on uPA production. No increase in FGF-2-induced PI3-kinase activity was observed in proteins immunoprecipitated by anti-phosphotyrosine antibody. Although stable expression of deleted mutant p85alpha regulatory subunit, which lacks association with p110 catalytic subunit, in IBE cells showed no dominant negative effect, transient expression of dominant negative Ras inhibited FGF-2-mediated PI3-kinase activation. These results suggest that only activated Ras contributed the FGF-2-mediated PI3-kinase activation. In cells stably expressing mutant p85alpha subunit, FGF-2 efficiently induced uPA production. Taken together, activation of PI3-kinase by FGF-2 is Ras-dependent and results in down-regulation of uPA production.

Increase in hepatocyte growth factor receptor tyrosine kinase activity in renal carcinoma cells is associated with increased motility partly through phosphoinositide 3-kinase activation

Dysregulated cell motility is one of the major characteristics of invasion and metastatic potentials of malignant tumor cells. Here, we examined the hepatocyte growth factor (HGF)-induced cell motility of two human renal carcinoma cell lines, ACHN and VMRC-RCW. Scattering and migration was induced in ACHN in an HGF-dependent manner, whereas they were maintained in VMRC-RCW even in the absence of HGF. In VMRC-RCW, HGF receptor (HGFR) tyrosine kinase was constitutively active, and sequence analysis showed N375S, A1209G and V1290L mutations. However, transfection experiments using porcine aortic endothelial (PAE) cells demonstrated that no single mutation or combination of two or three mutations caused HGF-independent constitutive activation. Conversely, the expressed amount of receptor protein had a pivotal role in the basal kinase activity. With respect to downstream signaling molecules of HGFR in ACHN or VMRC-RCW, the Ras-MAPK pathway was downregulated, whereas phosphoinositide 3-kinase (PI3-kinase) was not further activated by HGF-treatment in VMRC-RCW cells. The PI3-kinase inhibitors, wortmannin and LY294002 strongly inhibited spontaneous migration of VMRC-RCW. One transfected PAE cell line with massive overexpression of HGFR demonstrated scattered morphology and increased PI3-kinase activity in association with increased motility, which was partially inhibited by LY294002. Taken together, our results indicate that the overexpression of HGFR causes increase in cellular motility and PI3-kinase shows the important contribution on the increased motility of renal carcinoma cells.

Direct interaction between the cytoplasmic tail of ADAM 12 and the Src homology 3 domain of p85alpha activates phosphatidylinositol 3-kinase in C2C12 cells

ADAM 12, a member of the ADAM family of transmembrane metalloprotease-disintegrins, has been implicated previously in the differentiation of skeletal myoblasts. In the present study, we show that the cytoplasmic tail of mouse ADAM 12 interacts in vitro and in vivo with the Src homology 3 domain of the p85alpha regulatory subunit of phosphatidylinositol (PI) 3-kinase. By site-directed mutagenesis, we have identified three p85alpha-binding sites in ADAM 12 involving PXXP motifs located at amino acids 825-828, 833-836, and 884-887. Using green fluorescent protein (GFP)-pleckstrin homology (PH) domain fusion protein as a probe for PI 3-kinase lipid products, we have further demonstrated that expression of ADAM 12 in C2C12 cells resulted in translocation of GFP-PH to the plasma membrane. This suggests that transmembrane ADAM 12, by providing docking sites for the Src homology 3 domain of p85alpha, activates PI 3-kinase by mediating its recruitment to the membrane. Because PI 3-kinase is critical for terminal differentiation of myoblasts, and because expression of ADAM 12 is up-regulated at the onset of the differentiation process, ADAM 12-mediated activation may constitute one of the regulatory mechanisms for PI 3-kinase during myoblast differentiation.

Insulin-mediated GLUT4 translocation is dependent on the microtubule network

The GLUT4 facilitative glucose transporter is recruited to the plasma membrane by insulin. This process depends primarily on the exocytosis of a specialized pool of vesicles containing GLUT4 in their membranes. The mechanism of GLUT4 vesicle exocytosis in response to insulin is not understood. To determine whether GLUT4 exocytosis is dependent on intact microtubule network, we measured insulin-mediated GLUT4 exocytosis in 3T3-L1 adipocytes in which the microtubule network was depolymerized by pretreatment with nocodazole. Insulin-mediated GLUT4 translocation was inhibited by more than 80% in nocodazole-treated cells. Phosphorylation of insulin receptor substrate 1 (IRS-1), activation of IRS-1 associated phosphatidylinositide 3-kinase, and phosphorylation of protein kinase B/Akt-1 were not inhibited by nocodazole treatment indicating that the microtubule network was not required for proximal insulin signaling. An intact microtubule network is specifically required for insulin-mediated GLUT4 translocation since nocodazole treatment did not affect insulin-mediated GLUT1 translocation or adipsin secretion. By using in vitro microtubule binding, we demonstrated that both GLUT4 vesicles and IRS-1 bind specifically to microtubules, implicating microtubules in both insulin signaling and GLUT4 translocation. Vesicle binding to microtubules was not mediated through direct binding of GLUT4 or insulin-responsive aminopeptidase to microtubules. A model microtubule-dependent translocation of GLUT4 is proposed.

Epidermal growth factor-induced phosphatidylinositol 3-kinase activation and DNA synthesis. Identification of Grb2-associated binder 2 as the major mediator in rat hepatocytes

In previous work we showed that the phosphatidylinositol 3-kinase (PI3-kinase), not the mitogen-activated protein kinase, pathway is necessary and sufficient to account for insulin- and epidermal growth factor (EGF)-induced DNA synthesis in rat hepatocytes. Here, using a dominant-negative p85, we confirmed the key role of EGF-induced PI3-kinase activation and sought to identify the mechanism by which this is effected. Our results show that EGF activates PI3-kinase with a time course similar to that of the association of p85 with three principal phosphotyrosine proteins (i. e. PY180, PY105, and PY52). We demonstrated that each formed a distinct p85-associated complex. PY180 and PY52 each constituted about 10% of EGF-activated PI3-kinase, whereas PY105 was responsible for 80%. PY105 associated with Grb2 and SHP-2, and although it behaved like Gab1, none of the latter was detected in rat liver. We therefore cloned a cDNA from rat liver, which was found to be 95% homologous to the mouse Grb2-associated binder 2 (Gab2) cDNA sequence. Using a specific Gab2 antibody, we demonstrated its expression in and association with p85, SHP-2, and Grb2 upon EGF treatment of rat hepatocytes. Gab2 accounted for most if not all of the PY105 species, since immunoprecipitation of Gab2 with specific antibodies demonstrated parallel immunodepletion of Gab2 and PY105 from the residual supernatants. We also found that the PI3-kinase activity associated with Gab2 was totally abolished by dominant negative p85. Thus, Gab2 appears to be the principal EGF-induced PY protein recruiting and activating PI3-kinase and mitogenesis.

Negative regulation of PI 3-kinase by Ruk, a novel adaptor protein

Class I(A) phosphatidylinositol 3-kinase (PI 3-kinase) is a key component of important intracellular signalling cascades. We have identified an adaptor protein, Ruk(l), which forms complexes with the PI 3-kinase holoenzyme in vitro and in vivo. This interaction involves the proline-rich region of Ruk and the SH3 domain of the p85 alpha regulatory subunit of the class I(A) PI 3-kinase. In contrast to many other adaptor proteins that activate PI 3-kinase, interaction with Ruk(l) substantially inhibits the lipid kinase activity of the enzyme. Overexpression of Ruk(l) in cultured primary neurons induces apoptosis, an effect that could be reversed by co-expression of constitutively activated forms of the p110 alpha catalytic subunit of PI 3-kinase or its downstream effector PKB/Akt. Our data provide evidence for the existence of a negative regulator of the PI 3-kinase signalling pathway that is essential for maintaining cellular homeostasis. Structural similarities between Ruk, CIN85 and CD2AP/CMS suggest that these proteins form a novel family of adaptor molecules that are involved in various intracellular signalling pathways.

Backbone dynamics of the C-terminal SH2 domain of the p85alpha subunit of phosphoinositide 3-kinase: effect of phosphotyrosine-peptide binding and characterization of slow conformational exchange processes

The backbone dynamics of the C-terminal SH2 domain from the regulatory subunit p85alpha (p85alpha C-SH2) of phosphoinositide 3-kinase has been investigated in the absence of, and in complex with, a high-affinity phosphotyrosine-containing peptide ligand derived from the platelet-derived growth-factor receptor. (15)N R(1) and R(2) relaxation rates and steady-state [(1)H]-(15)N NOE values were measured by means of (1)H-(15)N correlated two-dimensional methods and were analyzed within the framework of the model-free formalism. Several residues in the BC loop and in the neighbouring secondary structural elements display fast local dynamics in the absence of phosphotyrosine peptide ligand as evidenced by below-average [(1)H]-(15)N NOE values. Furthermore, residue Gln41 (BC3) displays conformational exchange phenomena as indicated by an above-average R(2) relaxation rate. Upon binding of the phosphotyrosine peptide, the NOE values increase to values observed for regular secondary structure and the exchange contribution to the R(2) relaxation rate for Gln41 (BC3) vanishes. These observations indicate a loss of backbone flexibility upon ligand binding. Substantial exchange contributions for His56 (betaD4) and Cys57 (betaD5), which are known to make important interactions with the ligand, are attenuated upon ligand binding. Several residues in the betaD'-FB region and the BG loop, which contribute to the ligand binding surface of the protein, exhibit exchange terms which are reduced or vanish when the ligand is bound. Together, these observations suggest that ligand binding is accompanied by a loss of conformational flexibility on the ligand binding face of the protein. However, comparison with other SH2 domains reveals an apparent lack of consensus in the changes in dynamics induced by ligand binding. Exchange rates for individual residues were quantified in peptide-complexed p85alpha C-SH2 from the dependence of the exchange contributions on the CPMG delay in an R(2) series and show that peptide-complexed p85alpha C-SH2 is affected by multiple conformational exchange processes with exchange rate constants from 10(2) s(-1) to 7.10(3) s(-1). Mapping of the exchange-rate constants on the protein surface show a clustering of residues with similar exchange-rate constants and suggests that clustered residues are affected by a common predominant exchange process.

Expression of a prenylation-deficient Rab4 interferes with propagation of insulin signaling through insulin receptor substrate-1

Rab proteins are small GTP-binding proteins of the Ras superfamily that function in the regulation of vesicle transport processes. The Rab4 isoform has been implicated in insulin action. For instance, overexpression of a prenylation-deficient form of Rab4 has been shown to inhibit insulin-dependent GLUT4 translocation. Other steps affected by Rab4 in the cascade of events resulting from insulin receptor activation have not been elucidated. In the present studies, we measured effects on insulin-signaling proteins in 3T3-L1 adipocytes transiently expressing cytoplasmic forms of Rab4 and Rab5. Expression of a mutant Rab4 lacking a prenylation site resulted in reduced insulin-dependent phosphorylation ofcytoplasmic and internal membrane-associated insulin receptor substrate-1, leading to decreased insulin receptor substrate-1-associated phosphatidylinositol 3'-OH kinase activation and decreased Akt activation. These effects were not observed upon introduction of a similar mutant form of Rab5. These data indicate that Rab4 or a Rab4-associated protein is involved at one or more steps in propagating the insulin signal, in addition to any role it may play in the regulation of GLUT4 vesicle translocation. Our results support models of insulin signaling in which regulation of internal membrane trafficking plays a role in transduction of the insulin signal.

SYK is upstream of phosphoinositide 3-kinase in B cell receptor signaling

We have recently demonstrated that the D3-phosphoinositide phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P(3)) is critical for producing sustained calcium signals through its role in promoting the function of TEC family tyrosine kinases such as Bruton's tyrosine kinase. Although PtdIns-3,4,5-P(3) can potentially be synthesized by any of several types of phosphoinositide 3-kinases (PI3Ks), B cell receptor (BCR)-induced PtdIns-3,4,5-P(3) production is thought to occur primarily through the activation of the class Ia (p85/p110) PI3Ks. This process has been proposed to be mediated by an interaction between the Src family kinase LYN and the p85 subunit of PI3K and/or through p85 membrane recruitment mediated by CBL and/or CD19. However, calcium signaling and other PI3K-dependent signals are relatively preserved in a LYN kinase-deficient B lymphocyte cell line, suggesting that an alternative pathway for PI3K activation exists. As SYK/ZAP70 kinases are upstream from many BCR-initiated signaling events, we directly analyzed SYK-dependent accumulation of both PtdIns-3,4,5-P(3) and PtdIns-3,4-P(2) in B cell receptor signaling using both dominant negative and genetic knockout approaches. Both methods indicate that SYK is upstream of, and necessary for, a significant portion of BCR-induced PtdIns-3,4, 5-P(3) production. Whereas CD19 does not appear to be involved in this SYK-dependent pathway, the SYK substrate CBL is likely involved as the dominant negative SYK markedly attenuates CBL tyrosine phosphorylation and completely blocks the BCR-dependent association of CBL with p85 PI3K.

Structure and function of phosphoinositide 3-kinases

Phosphoinositide kinases (PI3Ks) play an important role in mitogenic signaling and cell survival, cytoskeletal remodeling, metabolic control and vesicular trafficking. Here we summarize the structure-function relationships delineating the activation process of class I PI3Ks involving various domains of adapter subunits, Ras, and interacting proteins. The resulting product, PtdIns(3,4,5)P3, targets Akt/protein kinase B (PKB), Bruton's tyrosine kinase (Btk), phosphoinositide-dependent kinases (PDK), integrin-linked kinase (ILK), atypical protein kinases C (PKC), phospholipase Cgamma and more. Surface receptor-activated PI3Ks function in mammals, insects, nematodes and slime mold, but not yeast. While many members of the class II family have been identified and characterized biochemically, it is presently unknown how these C2-domain containing PI3Ks are activated, and which PI substrate they phosphorylate in vivo. PtdIns 3-P is produced by Vps34p/class III PI3Ks and operates via the PtdIns 3-P-binding proteins early endosomal antigen (EEA1), yeast Vac1p, Vps27p, Pip1p in lysosomal protein targeting. Besides the production of D3 phosphorylated lipids, PI3Ks have an intrinsic protein kinase activity. For trimeric GTP-binding protein-activated PI3Kgamma, protein kinase activity seems to be sufficient to trigger mitogen-activated protein kinase (MAPK). Recent disruption of PI3K genes in slime mold, Caenorhabditis elegans, Drosophila melanogaster and mice further underlines the importance of PI3K signaling systems and elucidates the role of PI3K signaling in multicellular organisms.

Regulation of the p85/p110alpha phosphatidylinositol 3'-kinase. Distinct roles for the n-terminal and c-terminal SH2 domains

Our previous studies on the p85/p110alpha phosphatidylinositol 3-kinase showed that the p85 regulatory subunit inhibits the p110alpha catalytic subunit, and that phosphopeptide activation of p85/p110alpha dimers reflects a disinhibition of p110alpha (Yu, J., Zhang, Y., McIlroy, J., Rordorf-Nikolic, T., Orr, G. A., and Backer, J. M. (1998) Mol. Cell. Biol. 18, 1379-1387). We now define the domains of p85 required for inhibition of p110alpha. The iSH2 domain of p85 is sufficient to bind p110alpha but does not inhibit it. Inhibition of p110alpha requires the presence of the nSH2 domain linked to the iSH2 domain. Phosphopeptides increase the activity of nSH2/iSH2-p110alpha dimers, demonstrating that the nSH2 domain mediates both inhibition of p110alpha and disinhibition by phosphopeptides. In contrast, phosphopeptides did not increase the activity of iSH2/cSH2-p110alpha dimers, or dimers composed of p110alpha and an nSH2/iSH2/cSH2 construct containing a mutant nSH2 domain. Phosphopeptide binding to the cSH2 domain increased p110alpha activity only in the context of an intact p85 containing both the nSH2 domain and residues 1-322 (the SH3, proline-rich and breakpoint cluster region-homolgy domains). These data suggest that the nSH2 domain of p85 is a direct regulator of p110alpha activity. Regulation of p110alpha by phosphopeptide binding to the cSH2 domain occurs by a mechanism that requires the additional presence of the nSH2 domain and residues 1-322 of p85.

Stimulation of mitogenesis by a cell-permeable PI 3-kinase binding peptide

The binding of small phosphopeptides to the SH2 domains of the p85 regulatory subunit of PI 3-kinase can activate the enzyme in vitro. In the present study a cell-permeable peptide that binds specifically to the SH2 domains of p85 has been evaluated for its ability to stimulate a mitogenic response in the C2 muscle cell line. This peptide, in contrast to four other SH2-binding peptides, was as effective as serum, EGF, and FGF at stimulating entry into S-phase. The response to the p85 binding peptide, but not FGF, was inhibited by wortmannin and rapamycin, indicating that the peptide activates the PI 3-kinase/S6 kinase signalling pathway. The peptide response was not inhibited by the MEK inhibitor (PD098059) and did not stimulate Erk phosphorylation. Thus, there would appear to be no direct cross-talk between the pathway activated by the p85 binding peptide and the p42/p44 MAPK cascade.

Phosphorylation of the Grb2- and phosphatidylinositol 3-kinase p85-binding p36/38 by Syk in Lck-negative T cells

Activation of the mitogen-activated protein kinase (MAPK) pathway by the T-cell antigen receptor (TCR) in T cells involves a positive role for phosphatidylinositol 3-kinase (PI3K) activity. We recently reported that over-expression of the Syk protein tyrosine kinase in the Lck-negative JCaM1 cells enabled the TCR to induce a normal activation of the Erk2 MAPK and enhanced transcription of a reporter gene driven by the nuclear factor of activated T cells and AP-1. Because this system allows us to analyse the targets for Syk in receptor-mediated signalling, we examined the role of PI3K in signalling events between the TCR-regulated Syk and the downstream activation of Erk2. We report that inhibition of PI3K by wortmannin or an inhibitory p85 construct, p85deltaiSH2, reduced the TCR-induced Syk-dependent activation of Erk2, as well as the appearance of phospho-Erk and phospho-Mek. At the same time, expression of Syk resulted in the activation-dependent phosphorylation of three proteins that bound to the src homology 2 (SH2) domains of PI3K p85. The strongest of these bands had an apparent molecular mass of 36-38 kDa on SDS gels, and it was quantitatively removed from the lysates by adsorption to a fusion protein containing the SH2 domain of Grb2. The appearance of this band was Syk dependent, and it was seen only upon triggering of the TCR complex. Thus, p36/38 was phosphorylated by Syk or a Syk-regulated kinase, and this protein may provide a link to the recruitment and activation of PI3K, as well as to the Ras-MAPK pathway, in TCR-triggered T cells.

Solution structure of the C-terminal SH2 domain of the p85 alpha regulatory subunit of phosphoinositide 3-kinase

Heterodimeric class IA phosphoinositide 3-kinase (PI 3-kinase) plays a crucial role in a variety of cellular signalling events downstream of a number of cell-surface receptor tyrosine kinases. Activation of the enzyme is effected in part by the binding of two Src homology-2 domains (SH2) of the 85 kDa regulatory subunit to specific phosphotyrosine-containing peptide motifs within activated cytoplasmic receptor domains. The solution structure of the uncomplexed C-terminal SH2 (C-SH2) domain of the p85 alpha subunit of PI 3-kinase has been determined by means of multinuclear, double and triple-resonance NMR experiments and restrained molecular-dynamics simulated-annealing calculations. The solution structure clearly indicates that the uncomplexed C-SH2 domain conforms to the consensus polypeptide fold exhibited by other SH2 domains, with an additional short helical element at the N terminus. In particular, the C-SH2 structure is very similar to both the p85 alpha N-terminal SH2 domain (N-SH2) and the Src SH2 domain with a root mean square difference (rmsd) for 44 C alpha atoms of 1.09 and 0.89 A, respectively. The canonical BC, EF and BG loops are less well-defined by the experimental restraints and show greater variability in the ensemble of C-SH2 conformers. The lower level of definition in these regions may reflect the presence of conformational disorder, an interpretation supported by the absence or broadening of backbone and side-chain NMR resonances for some of these residues. NMR experiments were performed, where C-SH2 was titrated with phosphotyrosine-containing peptides corresponding to p85 alpha recognition sites in the cytoplasmic domain of the platelet-derived growth-factor receptor. The ligand-induced chemical-shift perturbations indicate the amino-acid residues in C-SH2 involved in peptide recognition follow the pattern predicted from homologous complexes. A series of C-SH2 mutants was generated and tested for phosphotyrosine peptide binding by surface plasmon resonance. Mutation of the invariant Arg36 (beta B5) to Met completely abolishes phosphopeptide binding. Mutation of each of Ser38, Ser39 or Lys40 in the BC loop to Ala reduces the affinity of C-SH2 for a cognate phosphopeptide, as does mutation of His93 (BG5) to Asn. These effects are consistent with the involvement of the BC loop and BG loops regions in ligation of phosphopeptide ligands. Mutation of Cys57 (beta D5) in C-SH2 to Ile, the corresponding residue type in the p85 alpha N-SH2 domain, results in a change in peptide binding selectivity of C-SH2 towards that demonstrated by p85 alpha N-SH2. This pattern of p85 alpha phosphopeptide binding specificity is interpreted in terms of a model of the p85 alpha/PDGF-receptor interaction.

Modification of phosphatidylinositol 3-kinase SH2 domain binding properties by Abl- or Lck-mediated tyrosine phosphorylation at Tyr-688

In cells expressing the oncogenic Bcr-Abl tyrosine kinase, the regulatory p85 subunit of phosphatidylinositol 3-kinase is phosphorylated on tyrosine residues. We report that this phosphorylation event is readily catalyzed by the Abl and Lck protein-tyrosine kinases in vitro, by Bcr-Abl or a catalytically activated Lck-Y505F in co-transfected COS cells, and by endogenous kinases in transfected Jurkat T cells upon triggering of their T cell antigen receptor. Using these systems, we have mapped a major phosphorylation site to Tyr-688 in the C-terminal SH2 domain of p85. Tyrosine phosphorylation of p85 in vitro or in vivo was not associated with detectable change in the enzymatic activity of the phosphatidylinositol 3-kinase heterodimer, but correlated with a strong reduction in the binding of some, but not all, phosphoproteins to the SH2 domains of p85. This provides an additional candidate to the list of SH2 domains regulated by tyrosine phosphorylation and may explain why association of phosphatidylinositol 3-kinase with some cellular ligands is transient or of lower stoichiometry than anticipated.

Interaction of wild type and dominant-negative p55PIK regulatory subunit of phosphatidylinositol 3-kinase with insulin-like growth factor-1 signaling proteins

In a first series of experiments done in the yeast two-hybrid system, we investigated the nature of protein-protein interaction between the regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase), p55PIK, and several of its potential signaling partners. The region between the Src homology 2 (SH2) domains of p55PIK bound to the NH2 terminus region of p110alpha, as previously shown for p85alpha. Moreover, we found that the insulin-like growth factor-1 receptor (IGF-IR) bound to p55PIK; the interaction occurred at the receptor tyrosine 1316 and involved both p55PIK SH2 domains. Interaction between p55PIK and IGF-IR was seen not only in the yeast two-hybrid system, but also using in vitro binding and coimmunoprecipitation of lysates from IGF-1 stimulated 293 cells overexpressing p55PIK. Further, IGF-I stimulation of these cells led to tyrosine phosphorylation of p55PIK. In 293 cells association of p55PIK with insulin receptor substrate-1 and with IGF-IR was dependent on PI 3-kinase, since it was increased by wortmannin, an inhibitor of PI 3-kinase. Further, by deleting amino acids 203-217 of p55PIK inter-SH2 domain, we engineered a p55PIK mutant unable to bind to the p110alpha catalytic subunit of PI 3-kinase. This mutant had a dominant-negative action on insulin-stimulated glucose transport, since insulin's effect on Glut 4 myc translocation was inhibited in adipocytes expressing mutant p55PIK. Importantly, this dominant-negative mutant was more efficient than wild type p55PIK in associating to IGF-IR and insulin receptor substrate-1 in 293 cells. Taken together, our results show that p55PIK interacts with key elements in the IGF-I signaling pathway, and that these interactions are negatively modulated by PI 3-kinase itself, providing circuitry for regulatory feedback control.

Site-directed mutagenesis and yeast two-hybrid studies of the insulin and insulin-like growth factor-1 receptors: the Src homology-2 domain-containing protein hGrb10 binds to the autophosphorylated tyrosine residues in the kinase domain of the insulin receptor

To characterize the structural basis for the interaction between hGrb10 and the insulin receptor and the insulin-like growth factor-1 receptor, different mutant receptors containing a segment of deletion in either the juxtamembrane domain or in the C terminus of the receptors, or containing tyrosine-to-phenylalanine point mutations in these regions of the insulin receptor, were generated. Yeast two-hybrid and in vitro binding studies of the interaction between the mutant receptors and hGrb10 revealed that tyrosine residues in these regions are not essential for the binding of hGrb10. To further identify the binding site for hGrb10, all conserved tyrosine residues in the kinase domain of the insulin receptor were replaced with either phenylalanine or alanine by site-directed mutagenesis. Mutations of all tyrosine residues in this region, except at positions 1162/1163, did not inhibit the binding of the receptor to hGrb10. The binding of the Src homology 2 domain of hGrb10 to the receptors was significantly enhanced in the presence of an intact pleckstrin homology domain. Our findings suggest that, unlike other Src homology 2 domain-containing proteins, hGrb10 binds to the autophosphorylated tyrosine residues in the kinase domain of the insulin receptor, and the pleckstrin homology domain plays an important role in hGrb10/receptor interaction. Because the autophosphorylated tyrosine residues are critical for the autophosphorylation and kinase activity of the receptor, the binding of hGrb10 at these sites may suggest a role for the protein in the transduction or regulation of insulin receptor signaling.

Involvement of phosphatidylinositol 3-kinase in NFAT activation in T cells

Phosphatidylinositol 3-kinase (PI3-K) has been implicated in the regulation of cell proliferation in many cell types. We have previously shown that in T cells the PI3-K inhibitor, wortmannin, interferes with activation of the mitogen-activated kinase, Erk2, after T cell receptor (TcR) stimulation. To further explore the involvement of PI3-K in T cell activation, we created a set of potentially dominant negative PI3-K constructs comprising individual or tandem domains of the regulatory p85 subunit and tested their effect on downstream signaling events like Erk2 activation and transcription from an NFAT (nuclear factor of activated T cells) element taken from the interleukin-2 promoter. Following TcR stimulation, activation of Erk2 was only inhibited by a previously described truncated form of p85 that cannot bind the catalytic subunit, but not by other constructs of p85. In contrast, several mutant p85 alleles had dramatic effects on NFAT activation. Most interestingly, the N-terminal SH2 domain had an inhibitory effect, whereas a mutant p85 containing only the two SH2 domains enhanced basal NFAT activity in a Ras-dependent manner. Ionomycin induced synergistic activation of NFAT in cells expressing p85 mutants that contained the C-terminal SH2 domain. Analysis of phosphotyrosine-containing proteins bound to truncated p85 constructs revealed cooperative binding of the two SH2 domains but no apparent differences between the N- and C-terminal SH2 domains. Wortmannin did not interfere with NFAT activation, although it inhibited PI3-K and Erk2 activation in the same experiment. These results suggest that PI3-K is involved in NFAT activation through a complex adaptor function of its regulatory subunit and that its lipid kinase activity is dispensable for this effect.

Specific increase in p85alpha expression in response to dexamethasone is associated with inhibition of insulin-like growth factor-I stimulated phosphatidylinositol 3-kinase activity in cultured muscle cells

The stimulation of phosphatidylinositol (PI) 3-kinase by insulin-like growth factor I (IGF-I) in L6 cultured skeletal muscle cells is inhibited by the glucocorticoid dexamethasone. The objective of this study was to investigate the mechanism of dexamethasone action by determining its effects on the expression of the p85alpha and p85beta regulatory subunit isoforms of PI 3-kinase, their coupling with the p110 catalytic subunit, and their association with insulin receptor substrate 1 (IRS-1) in response to IGF-I stimulation. Dexamethasone induced a 300% increase in p85alpha protein content in the L6 cultured myoblast cell line, whereas it increased p110 content by only 38% and had no effect on p85beta. The increase in p85alpha protein was associated with a coordinate increase in p85alpha mRNA. Stimulation with IGF-I induced the association of p85alpha and p85beta with IRS-1, and this was accompanied by increased amounts of the p110 catalytic subunit and markedly increased PI 3-kinase activity in IRS-1 immunoprecipitates. In cells treated with dexamethasone, greater amounts of p85alpha and lower amounts of p85beta, respectively, were found in IRS-1 immunoprecipitates, such that the alpha/beta ratio was markedly higher than in control cells. In spite of the increase in both total and IRS-1-associated p85alpha following dexamethasone treatment, IRS-1-associated p110 catalytic subunit and PI 3-kinase activity were decreased by approximately 50%. Thus, dexamethasone induces a specific increase in expression of the p85alpha regulatory subunit that is not associated with a coordinate increase in the p110 catalytic subunit of PI 3-kinase. As a consequence, in dexamethasone-treated cells, p85alpha that is not coupled with p110 competes with both p85alpha.p110 and p85beta.p110 complexes for association with IRS-1, leading to increased p85alpha but decreased p85beta, p110, and PI 3-kinase activity in IRS-1 immunoprecipitates.

Inhibition of MG-63 cell proliferation and PDGF-stimulated cellular processes by inhibitors of phosphatidylinositol 3-kinase

Studies on a platelet-derived growth factor (PDGF) responsive osteosarcoma cell line, MG-63, were initiated to determine the effects of phosphatidylinositol (Ptdlns) 3-kinase inhibitors on serum-stimulated cell proliferation and PDGF-stimulated DNA replication, actin rearrangements, or Ptdlns 3-kinase activity. In a dose-dependent manner, the fungal metabolite wortmannin and a quercetin derivative, LY294002 (2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one), inhibited serum-stimulated MG-63 cell proliferation. The mitogenic effects of PDGF on MG-63 cells, as determined by incorporation of [3H]-thymidine, were also substantially inhibited in the presence of 0.10 microM wortmannin or 10 microM Ly294002. Furthermore, MG-63 cells stimulated by PDGF form distinct actin-rich, finger-like membrane projections which are completely inhibited by either 0.10 microM wortmannin or 10 microM LY294002. At these same concentrations, wortmannin and LY294002 were also effective at reducing levels of phosphatidylinositol 3-phosphate in PDGF-stimulated MG-63 cells. Treatment of these cells with increasing concentrations of wortmannin reduced the level of PDGF stimulated tyrosine phosphorylation of the PDGF receptor but did not significantly affect the amount of the Ptdlns 3-kinase regulatory subunit, p85, associated with the receptor. Additionally, pretreatment of cells with 0.250 microM wortmannin followed by stimulation with PDGF resulted in a slightly reduced level of receptor autokinase activity; however, similar treatment with 50 microM LY294002 did not affect the level of autokinase activity. These results demonstrate the effects of two different Ptdlns 3-kinase inhibitors on serum- and PDGF-stimulated MG-63 cell proliferation and PDGF-stimulated morphological changes and suggest a greater role for Ptdlns 3-kinase in these processes.

Cloning and mutagenesis of the p110 alpha subunit of human phosphoinositide 3'-hydroxykinase

Activation of phosphoinositide 3'-hydroxykinase (P13K) is required for mitogenic signal transduction by several growth factors and oncogenes. P13K is a heterodimer consisting of a p85 regulatory subunit and a p110 catalytic subunit. In the current study, we report the cloning and characterization of the p110 alpha catalytic subunit of human P13K. This clone is highly homologous (> 99% amino acid identity) to bovine brain p110 alpha, but contains 10 amino acid differences from the human p110 alpha sequence previously reported. Comparison of this sequence with known Ser/Thr kinases and p110 homologs highlighted several conserved residues within the putative kinase domain. Mutational analysis of these residues (Asp915, (Asp933 + Phe934)) yielded P13K mutants with virtually complete loss of phosphoinositide phosphorylating activity. Expression of the wild-type p110 alpha protein in CHO cells is sufficient to activate the serum response element derived from the promoter of c-fos, an immediate early gene product. In contrast, the catalytically impaired p110 alpha mutants as well as the p85 alpha subunit of P13K were inactive in the fos assay. These studies suggest that the mitogenic signal transduction pathway mediated by P13K is dependent upon the enzymatic activity of the p110 alpha subunit of P13K.

Protein-protein interactions in the yeast PKC1 pathway: Pkc1p interacts with a component of the MAP kinase cascade

The two-hybrid system for the identification of protein-protein interactions was used to screen for proteins that interact in vivo with the Saccharomyces cerevisiae Pkc1 protein, a homolog of mammalian protein kinase C. Four positive clones were isolated that encoded portions of the protein kinase Mkk1, which acts downstream of Pkc1p in the PKC1-mediated signalling pathway. Subsequently, Pkc1p and the other PKC1 pathway components encoding members of a MAP kinase cascade, Bck1p (a MEKK), Mkk1p, Mkk2p (two functionally homologous MEKs), and Mpk1p (a MAP kinase), were tested pairwise for interaction in the two-hybrid assay. Pkc1p interacted specifically with small N-terminal deletions of Mkk1p, and no interaction between Pkc1p and any of the other known pathway components could be detected. Interaction between Pkc1p and Mkk1p, however, was found to be independent of Mkk1p kinase activity. Bck1p was also found to interact with Mkk1p and Mkk2p, and the interaction required only the predicted C-terminal catalytic domain of Mkk1p. Furthermore, we detected protein-protein interactions between two Bck1p molecules via their N-terminal regions. Finally, Mkk2p and Mpk1p also interacted in the two-hybrid assay. These results suggest that the members of the PKC1-mediated MAP kinase cascade form a complex in vivo and that Pkc1p is capable of directly interacting with at least one component of this pathway.

Controlled overexpression of selected domains of the P85 subunit of phosphatidylinositol 3-kinase reverts v-Ha-Ras transformation

Selected domains of the regulatory p85 subunit of phosphatidylinositol 3-kinase have been expressed under the control of the tetracycline transactivator in NIH 3T3 fibroblasts transformed by the v-Ha-Ras oncogene. The domains expressed were the SH3 domain, the BCR homology domain, the region between the two SH2 domains which contains the p110 binding site (the inter SH2 (IS) domain), and the C-terminal (CT) domain (containing both SH2 domains and the IS domain). The levels of IS or SH3 domain expressed in the presence of tetracycline were sufficient to reverse the transforming effects of v-Ha-Ras, and no further inhibition of proliferation was observed when expression was increased 7-fold by removal of tetracycline. In contrast inhibition of proliferation by the CT domain was observed only when the level of expression was increased 5-fold by removal of tetracycline. Overexpression of the BCR domain of p85 had no effect on v-Ha-Ras transformation. Expression of the IS domain disrupted the interaction of the p85 regulatory subunit with the p110 catalytic subunit. These results indicate that the association of the p85 subunit of PI 3-kinase with the p110 subunit is necessary for v-Ha-Ras-induced transformation in NIH 3T3 cells.

Cpk is a novel class of Drosophila PtdIns 3-kinase containing a C2 domain

We report the identification of a novel class of phosphatidylinositol (PtdIns) 3-kinases whose members contain C-terminal C2 domains. We have isolated Drosophila and murine genes (termed cpk and cpk-m respectively) by polymerase chain reaction amplification of cDNA libraries with degenerate primers corresponding to conserved regions of PtdIns kinases. The amino acid sequences of Cpk and Cpk-m are most similar to that of p110, a family of PtdIns 3-kinases that mediates the responses of cells to mitogenic stimuli. The Cpk and Cpk-m sequences are similar to a large, central region of p110, but differ from p110 at their N and C termini. The N termini of the Cpk proteins do not contain any recognizable protein motif, while the C termini contain "C2 domains," a feature unique among PtdIns kinases. Cpk has an intrinsic PtdIns kinase activity and can phosphorylate PtdIns and PtdIns-4-P, but not PtdIns(4,5)P2, at the D3 position of the inositol ring. Cpk is the first PtdIns 3-kinase identified with this particular substrate specificity. We have identified two potential Cpk-binding proteins, p90 and p190, and have determined that both Cpk and p190 may be tyrosine phosphorylated. This finding suggests that Cpk function may be regulated by tyrosine kinases.

Association of phosphatidylinositol 3-kinase, via the SH2 domains of p85, with focal adhesion kinase in polyoma middle t-transformed fibroblasts

Focal adhesion kinase (FAK), a non-receptor protein tyrosine kinase, becomes activated and phosphorylated on tyrosine in cells transformed with v-src. By cytoimmunofluorescence a sub-fraction of the p85 subunit of phosphoinositide 3-kinase (PI 3-kinase) localized in focal adhesion plaques. We examined the possibility that FAK associates with PI 3-kinase. In fibroblasts transformed with polyoma middle t, PI 3-kinase activity co-immunoprecipitated with pp125FAK using two different antibodies against this protein. PP125FAK from middle t-transformed cells associated with a glutathione-S-transferase fusion protein containing the 85-kDa subunit of phosphatidylinositol 3-kinase. Both of the SH2 domains and the SH3 domain of p85 also formed complexes with pp125FAK in vitro. Phosphopeptides that bind to the SH2 domains completely blocked the binding of full-length p85 to pp125FAK, while a peptide that binds to the SH3 domain was ineffective, indicating that the association between p85 and pp125FAK is mediated by the SH2 domains of p85.

Phosphoinositide 3-kinase gamma and p85/phosphoinositide 3-kinase in platelets. Relative activation by thrombin receptor or beta-phorbol myristate acetate and roles in promoting the ligand-binding function of alphaIIbbeta3 integrin

Platelets exposed to thrombin or thrombin receptor agonist peptide (SFLLRN) activate phospholipase C and protein kinase C (PKC), and accumulate 3-phosphorylated phosphoinositides (3-PPI) as a function of the activation and relocalization of two cytoskeletally-associated phosphoinositide 3-kinases (PI 3-K): p85/PI 3-K and PI 3-Kgamma. We now report that exposure of platelets to PKC-activating beta-phorbol myristate acetate (betaPMA) does not stimulate PI 3-Kgamma, but rather stimulates p85/PI 3-K, which associates with the cytoskeleton. Wortmannin is an inhibitor of both PI 3-Ks, known to act with more potency on p85/PI 3-K. betaPMA-stimulated 3-PPI accumulation is more sensitive to wortmannin (IC50 = 1.3 nM) than is SFLLRN- or thrombin-stimulated 3-PPI accumulation (IC50 = 10 nM). The activity of p85/PI 3-K in immunoprecipitates or in cytoskeletal fractions is inhibited more potently by exposure of platelets to wortmannin than is the activity of PI 3-Kgamma. betaPMA or SFLLRN promotes the conversion of platelet integrin alphaIIb/beta3 into a fibrinogen-binding form required for platelet aggregation. Activation of alphaIIb/beta3 in response to betaPMA or SFLLRN is inhibited by wortmannin with an IC50 of 1 nM in each case. Wortmannin inhibits neither activation of alphaIIb/beta3 by ligand-induced binding site antibody (anti-LIBS6 Fab) nor anti-LIBS6 Fab-induced platelet aggregation in the presence of fibrinogen, indicating that this type of "outside-in" signaling by alphaIIb/beta3 is largely PI 3-K-independent. We conclude that p85/PI 3-K, in preference to PI 3-Kgamma, contributes to activation of alphaIIb/beta3 when the thrombin receptor or PKC is stimulated.

Inhibition of phosphatidylinositol 3-kinase blocks T cell antigen receptor/CD3-induced activation of the mitogen-activated kinase Erk2

The production of 3-phosphorylated inositol phospholipids is implicated in regulation of cell growth and transformation. To explore the role of these lipids in T cell antigen receptor (TCR)/CD3-induced signaling, we have examined the effects of a specific phosphatidylinositol 3-kinase (PtdIns3K) inhibitor, wortmannin, and overexpression of two PtdIns3K constructs on the activation of down-stream effectors in anti-CD3 treated T cells. We report that treatment of cells with wortmannin blocked anti-CD3-induced activation of the mitogen-activation kinase Erk2 while not affecting phorbol-ester-induced Erk2 activation. An inactive analog of wortmannin, WM12, did not affect TCR/CD3-induced Erk2 activation, and wortmannin had no effect on the activity of Erk2 when added directly to the in vitro assays. Expression of a disruptive PtdIns3K construct also reduced Erk2 activation, while a construct that stimulates PtdIns3K enhanced the activation of Erk2. Receptor-induced activation of other Ser/Thr kinases, such as c-Raf, B-Raf, Mek1, Mek2, Mekk, was not affected by wortmannin. Our results suggest that the production of 3-phosphorylated inositol phospholipids is involved in the activation of Erk2, but does not regulate the enzymes that are thought to be upstream of Erk2.

Finding prospective partners in the library: the two-hybrid system and phage display find a match

The two-hybrid system uses the efficacy of yeast genetic assays to identify protein-protein interactions. It permits the rapid cloning of genes encoding products that interact with a given protein of interest. Also being developed are phage display methods that allow direct physical selection of binding proteins. These methods have significantly altered strategies for analysing signaling and regulatory pathways.